Genesis of the Bangbu gold deposit in the southern Tibet: Evidenced from in-situ sulfur isotopes and trace element compositions of pyrite

The Bangbu orogenic gold deposit in the North Himalaya of the southern Tibet contains more than 40 t Au at an average grade of 7.0 g/t. In this deposits, gold-bearing quartz veins were controlled by nearly E-trending Qusong-Cuogu-Zhemulang shear zone and occurred within the secondary faults which crosscut Late Triassic greenschist- facies rocks. To further understand the sulfur source and ore-forming process, we have conducted a compressive study of in-situ SIMS sulfur isotopes and LA-ICP-MS trace element compositions of two stages of pyrite at Bangbu. Early-stage pyrite (Py1) is coarse-grained (mostly 0.2–2 mm) and euhedral, and has gold concentrations of less than 0.3–54 ppm (mean of 20 ppm) and δ³⁴S values of 1.6–5.1‰ (mean of 3.2‰). Late-stage pyrite (Py2) is generally fine-grained (mostly 34S values of 0.9–5.2‰ (mean of 2.7‰). Gold occurs mainly as invisible refractory within Py1 and Py2, and to a lesser extent as native gold within quartz, pyrite, arsenopyrite and sphalerite. Sulfur for Bangbu gold mineralization was probably sourced from the Greater Himalayan crystalline complex. Release of ore-forming fluids was likely related to amphibolite-facies metamorphism during ~50–45 Ma. Ore fluids deposited Au-rich pyrite during early and late mineralization stage and precipitation of native gold was probably related to fluid boiling and/or remobilization of invisible gold within pyrite.